Thoughts on Building a Blender-Oriented PC
Note

Although this question is about hardware we need a canonical answer on this!


Canonical posts for hardware questions?
Answering vague support questions (once)?




Answer

Mike Pan has started a thread on BA and collects benchmark results in a sheet.
Be careful because the benchmark results may differ due to different Blender versions and Cycles performance has been optimized.

A few things to consider:

GPU

Cycles uses CUDA and is provided by NVIDIA only.
A single system designed for Cycles should be focused on maximum number of shaders kernels (CUDA cores) on the GPU. When you use shaders written in OSL the scene can only be rendered on the CPU.

Nvidia model numbers start with a digit for the generation 5,6,7 

 oldest    540 &lt; 550 &lt; 560 &lt; 570 &lt; 580 (fastest, 512 CUDA cores)
           640 &lt; 650 &lt; 660 &lt; 670 &lt; 680 (fastest, 1536 CUDA cores)
 newest    740 &lt; 750 &lt; 760 &lt; 770 &lt; 780 &lt; 780Ti (fastest, 2880 CUDA cores)


You could find something like 590, this means AFAIK the two 580's have been connected.

GPU RAM

If you use many big textures the RAM of a 1.5GiB card could be the limiting factor more is better.

CPU

Baking, preparation of the scene is calculated only by the CPU. According to the benchmarks I guess Intel is to prefer.

i3 2 Cores (slowest)
i5 4 Cores 
i7 4 Cores + Hyper-threading (fastest) Hyper-Threating adds approx. 10-20% of performance.


Server CPU

A Xeon-CPUs an E3-1231-v3 (4-cores) costs a bit more than an i5 but comes with Hyper-threading. . There also costly CPUs like E5 available Depending on the motherboard you can add 2 of them (Socket 2011) having hexa(6), octa(8) or deca(10) cores (plus hypertheading which adds ~10% performance) but they cost > 1.250$ (Sep. 2014).

System RAM


Should start at 8GiB, 16GiB would be suitable for most use cases.


Realated Resources


Blender Cycles performance benchmark - more CPU cores vs higher clock frequency benchmark
Cycles versions speed comparision


Some numbers from my machines for Mike Pan's BMW:

Blender v2.71

i5-3470@3.2 GHz 02:50.00
CUDA GeForce GTX 660 Ti 01:12.65

Xeon E3-1231@3.4 GHz 01:36.48
CUDA GeForce GTX 580 01:08.23

i7-2630QM@2.0 GHz 03:39.96
CUDA GeForce GTX 560M 03.00.87


You find a lot more results at:


Blenchmark benchmarks-overview